1. Field of the Invention
The present invention relates to an automatic focusing apparatus which is particularly suitable for an electronic endoscope.
2. Description of the Related Art
In recent years, an electronic endoscope having an automatic focusing apparatus has been developed. In a conventional automatic focusing apparatus, focus detection is carried out in such a way that when the evaluation value of the high frequency component of an image signal picked up by a CCD-type or CMOS-type image pickup device becomes maximum, i.e., when a high frequency component for a contour portion of an object image becomes maximum, it is deemed that the image is in focus. During a diagnosis using an electronic endoscope, as the object to be detected frequently changes in accordance with the endoscopic operation, hunting of the automatic focusing optical system occurs if the automatic focusing apparatus is continuously activated. Alternatively, in order to activate the automatic focusing apparatus only when automatic focusing is necessary, a separate switch must be provided to activate the automatic focusing apparatus. However, such a switch must be operated each time the automatic focusing apparatus is activated or deactivated.
An electronic endoscope having an automatic focusing apparatus is disclosed, for example, in References No. 1 through No. 4 below.                Reference No. 1: Japanese Kokai (Unexamined Patent Publication) No. 2001-154085        Reference No. 2: Japanese Kokai (Unexamined Patent Publication) No. 2000-5127        Reference No. 3: Japanese Kokai (Unexamined Patent Publication) No. 2000-197604        Reference No. 4: Japanese Kokai (Unexamined Patent Publication) No. 2000-342533        